The present invention generally relates to the field of particulate matter contamination in electronic packaging for computer systems, and more particularly to measuring the deliquescence relative humidity of dust.
The physical environment surrounding a printed circuit board (PCB) is defined by the temperature, humidity and gaseous and particulate contamination in the air. Environmental factors may cause PCBs to fail in two ways: First, electrical open circuits may result from corrosion, such as the corrosion of silver terminations in surface mount components. Second, electrical short circuits may be caused by (a) copper creep corrosion, (b) electrochemical reactions such as ion migration and cathodic-anodic filamentation or (c) settled, hygroscopic particulate matter contamination reducing the surface insulation resistance between closely spaced features on PCBs. In 2006, the European Union's Restriction of Hazardous Substances (RoHS) directive banning the use of lead in solders led to changes in PCB finishes and the elimination of lead from solders. These changes dramatically increased the PCB failure rates due to creep corrosion. Another common failure mode during this period was that of surface mount resistors suffering open circuits due to the corrosion of their silver terminations. Information technology (IT) equipment manufacturers have since learned to make their hardware robust against these two failure modes, which used to occur predominantly in geographies with high levels of sulfur-bearing gaseous contamination. The failure mode that is much more difficult to deal with and eliminate is that of the electrical short circuiting caused by the accumulated particulate matter in humid environments. The difficulty arises from the intermittent electrical nature of these particles and that the failure leaves no visible evidence besides the presence of deposited particulate matter.
The rapid expansion of the IT equipment market in the polluted geographies of Asia that have high levels of fine particulate matter in the ambient air and the increasing use of free cooling is introducing this new, often intermittent, short-circuit failure mode due to particulate matter. The source of particulate matter is both natural and anthropogenic. In terms of size, particulate matter may be divided into two categories: fine and coarse particles. Fine particles (<2.5 μm), such as those found in motor vehicle exhaust, diesel particulate matter (DPM), smoke and haze, are of two types: primary and secondary. The primary fine particles are directly emitted from a source, such as a forest fire, volcanoes, construction sites, unpaved roads, fields or smokestacks. The secondary fine particles, which make up most of the fine particulate pollution, are those formed as a result of photochemical reactions in the atmosphere. This is generally due to the presence of oxides of nitrogen and sulfur emitted from power plants, industries and automobiles. Sulfur dioxide and nitrogen dioxide interact with <0.1 m size carbonaceous material seed particles in a complex, multistep photochemical process to produce sulfuric and nitric acids. These acids are neutralized by ammonia from fertilizers, decay of biological materials and other sources to produce fine particles dominated by ammonium sulfate, ammonium hydrogen sulfate and ammonium nitrate. The majority of these secondary fine particles would be considered anthropogenic. Coarse particles, which are in the 2.5-15 μm size range, include sea salt, natural and artificial fibers, plant pollens, and wind-blown dust. Their sources include erosion of soil and minerals and flaking of biological materials.